Rat relaxin will be isolated in pure form in order to provide the hormone required for the study of its physiological properties. A comprehensive study of the levels of relaxin immunoreactivity present in the blood of rats throughout pregnancy will be conducted with a newly developed homologous rat relaxin radioimmunoassay. Relaxin immunoreactivity levels will be determined in pigs following surgical and pharmacological alterations which influence parturition. Additional studies will be conducted to determine the location of relaxin responsive tissues in the rat.